CARBOHYDRATES
General Information:
Carbohydrates are the most abundant class of organic compounds found in living organisms.
They originate as products of photosynthesis, an endothermic reductive condensation of
carbon dioxide requiring light energy and the pigment chlorophyll.
n CO2 + n H2O + energy
CnH2nOn + n O2
As noted here, the formulas of many carbohydrates can be written as carbon hydrates,
Cn(H2O)n, hence their name. The carbohydrates are a major source of metabolic energy,
both for plants and for animals that depend on plants for food. Aside from the sugars and
starches that meet this vital nutritional role, carbohydrates also serve as a structural
material (cellulose), a component of the energy transport compound ATP, recognition sites
on cell surfaces, and one of three essential components of DNA and RNA.
Carbohydrates are called saccharides or, if they are relatively small, sugars.
A- Simple Sugars
1- Contain the elements carbon, hydrogen, and oxygen.
- The name carbohydrate literally means water compounds of carbon.
- The general formula for simple sugars is Cn(H2O)n.
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- This class of compounds is better described as Polyhydroxy aldehydes and ketones.
- The simplest carbohydrates are glyceraldehyde and dihydroxyacetone.
O
HC
C
HO
O
H
CH2OH
glyceraldehyde
HOH2C
C
CH2OH
dihydroxyacetone
A - Methods of Classification:
- Several methods are used to classify carbohydrates.
1-One method of classification is based on whether the carbohydrate can be
broken down into smaller units.
Monosaccharides - cannot be broken down into smaller units by hydrolysis. Sometimes
called simple sugars.
Disaccharides - can be broken down (hydrolyzed) into two monosaccharide units.
Oligosaccharides - can be broken into three to six monosaccharide units.
Polysaccharides - composed of 7 or more mono-saccharide units.
2-Another method is based on the number of carbons found in a simple sugar.
- If it has three carbons it is called a triose.
- If it has four carbons it is called a tetrose.
- If it has five carbons it is called a pentose.
- If it has six carbons it is called a hexose.
3-Another method uses the kind of carbonyl group.
A- Aldose - a monosaccharide with an aldehyde group.
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O
HC
H
C
OH
H
C
OH
CH2OH
erythrose
B- Ketose - a monosaccharide with a ketone group.
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
fructose
- Usually combine the carbonyl classification and the number classification together.
H
O
C
H
H
C
OH
HO
C
H
H
C
H
C
O
C
H
C
CH2OH
OH
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
CH2OH
CH2OH
glucose
glyceraldehyde
aldotriose
aldohexose
CH2OH
fructose
ketohexose
B- Stereoconfigurations of simple sugars.
- Carbohydrates contain many stereocenters.
1- If the OH group is found on the right side of the carbon chain, the sugar is designated as
a D sugar.
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2- If the OH group is found on the left side of the chain of carbons, the sugar is designated
as an L sugar.
H
H
O
C
H
C OH
C
O
HO C H
CH2OH
CH2OH
D-glyceraldehyde
L-glyceraldehyde
D-aldotriose
L-aldotriose
CHO
H
C OH
CH2OH
D-glyceraldehyde
CHO
H
C OH
H
C OH
CHO
HO C H
H
C OH
CH2OH
CH2OH
D-erythrose
D-threose
CHO
HO C H
H C OH
H
C OH
H
C OH
H
C OH
H
C OH
CH2OH
CHO
HO C H
HO C H
H
H
C OH
C OH
CH2OH
D-mannose
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H
C OH
HO C H
H
H
C OH
C OH
CH2OH
D-glucose
H
CHO
HO
H
C H
C OH
H C OH
H
C OH
CH2OH
D-altrose
H
H
C OH
C OH
HO
C H
D-xylose
H
C OH
C OH
HO C H
HO C H
H C OH
HO C H
HO C H
H
H
C OH
CH2OH
D-allose
H
C OH
CH2OH
D-tallose
H
C OH
CH2OH
D-galactose
CHO
CHO
CHO
CHO
C OH
CH 2OH
CH2OH
D-lyxose
CHO
C OH
HO C H
HO C H
CH2OH
CHO
H
HO C H
D-ribose
D-arabinose
CHO
CHO
CHO
HO C H
H
C OH
HO C H
H
C OH
H
C OH
H
C OH
HO C H
H
CH2OH
CH2OH
D-idose
C OH
D-gulose
Organized by: Sharifa Al-Ghamdi& Huda Al-Shaibi
CH2OH
C
O
CH2OH
dihydroxyacetone
CH2OH
H
C
O
C
OH
CH2OH
D-erythrulose
CH2OH
CH2OH
C
O
C
O
HO
C
H
H
C
OH
H
C
OH
H
C
OH
CH2OH
D-ribulose
CH2OH
D-xylulose
CH2OH
CH2OH
CH2OH
CH2OH
C
O
C
O
C
O
C
O
HO
C
H
H
C
OH
HO
C
H
H
C
OH
HO
C
H
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
CH2OH
D-tagatose
CH2OH
D-sorbose
CH2OH
CH2OH
D-fructose
D-psicose
Cyclic Structures:
- Five membered sugar rings are known as furanose rings.
- Six membered sugar rings are known as pyranose rings.
H
C
O
H C OH
HO CH2
H
H C OH
H C OH
CH2OH
D-ribose
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H
O
H
H
H
+
OH
OH
HO CH2
OH
-D-ribofuranose
OH
O
H
H
H
OH
OH
-D-ribofuranose
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H
H
C
O
C OH
HO C H
H
C OH
H
C OH
CH2OH
D-glucose
HO CH2
H
OH
OH H
HO
+
OH
H OH
-D-glucopyranose
HO CH2
H
O OH
OH H
HO
H
H OH
-D-glucopyranose
Carbohydrate Anomers:
- Formation of either of the cyclic form has created a new stereocenter.
- These stereoisomeric ring forms of carbohydrates are called Anomers.
- Anomers are carbohydrates that differ by the stereo-configuration of the carbon involved
in ring formation.
- The greek letters α and β are used to describe the configuration about the ring forming
carbon.
- The α anomer always has the OH group oriented in a downward fashion on the anomeric
carbon of a D-sugar.
- The β anomer always has the OH group oriented in an upward fashion on the anomeric
carbon of a D-sugar.
Important Carbohydrates:
Monosaccharides
- composed of three to seven carbon atoms.
1- Glucose - most abundant hexose in our diet.
- The building block of complex carbohydrates.
- Component of the disaccharides: sucrose, maltose and lactose.
- Found in the polysaccharides: starch, cellulose and glycogen.
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CHO
H
CH2OH
C OH
H
HO C H
H
C OH
H
C OH
O
H
OH
OH
H
CH2OH
H,OH
H
OH
2- Galactose
- Found in the disaccharide, lactose.
- Found in the cellular membranes of the brain and nervous system.
- Galactose is the C-4 epimer of glucose.
CHO
CH2OH
H C OH
HO
HO C H
HO C H
O
H
OH
H
H
OH
H,OH
H
H C OH
CH2OH
3- Fructose
- Sweetest of the carbohydrates.
- Component of the disaccharide sucrose.
- Fructose is a keto sugar.
Disaccharides
- composed of two monosaccharide units.
1- Maltose - malt sugar.
Used in cereals, candies and the brewing of beverages.
Composed of two D-glucose sugars joined by an α-1,4 linkage.
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CH2OH
CH2OH
O H
H
H
OH
O OH
H
H
OH
H
H
O
OH
H
H
OH
H
OH
2-Lactose - milk sugar.
- Found in milk and milk products.
- Composed of one galactose and one glucose unit joined by a β-1,4 linkage.
CH 2OH
CH 2OH
OH
O
H
OH
O OH
H
H
OH
O
H
H
H
H
H
H
OH
OH
3- Sucrose - table sugar.
- Product of sugar cane and sugar beets.
- Composed of one glucose and one fructose unit.
- Linkage is at both anomeric carbons.
CH2OH
H
H
OH
CH2OH
O
O H
H
H
H
OH
OH
H
O
OH
H
OH
CH2OH
Polysaccharides
- composed of many (more than 10) monosaccharide units.
1- Cellulose:
Major structural material of plant cells.
Consists of many glucose units joined by β-1,4 linkages.
2- Starch:
Storage form of glucose found in rice wheat, potatoes, grains and cereals.
Consists of many glucose units joined by α-1,4 linkages.
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Maltose is the disaccharide starting material.
3- Glycogen:
Animal starch. Storage form of glucose found in the liver and muscle of animals.
Contains many highly branched glucose units.
Joined by α-1,4 linkages and branched by α-1,6 linkages.
4- Dextrin:
- Mixture of branched and un-branched soluble polysaccharides produced by partial
hydrolysis of starch by acids or amylases.
Reducing sugars:
Any sugar that contains either:
1-A free aldehyde group.
2- An α-hydroxy ketone group.
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3-A hemiacetal linkage
-The presence of any of these groups allows the carbohydrate to undergo easy oxidation.
- If the sugar gets oxidized it causes reduction.
- Thus the name “reducing sugar”.
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QUALITATIVE TESTS FOR CARBOHYDRATES
Experiment-1:
Molisch Test:
- It is the general test for all carbohydrates.
- All carbohydrates. Monosaccharides give a rapid positive test. Disaccharides and
polysaccharides react slower.
- The Molisch reagent dehydrates pentoses to form furfural (top reaction) and dehydrates
hexoses to form 5-hydroxymethyl furfural (bottom reaction). The furfurals further react
with
-naphthol present in the test reagent to produce a purple product.
→ Condensation with α-naphthol >>>> Purple ring.
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Method:
1ml test solution + 2 drops of α-naphthol >> mix well >> add conc. H2SO4 down the side of
the tube to form the ring at the interface of the two layers.
-ve
+ve
Experiment-2:
Fehling's Test:
- This test is used to differentiate between reducing and non reducing sugars.
- A reducing sugar reacts with Fehling's reagent in alkaline medium to form an orange to red
precipitate. Fehling's reagent is commonly used for reducing sugars but is known to be not
specific for aldehydes.
- Positive result is detected by reduction of the deep blue solution of cupric (II) to a red
precipitate of insoluble cuprous oxide (Cu2O).
- The sucrose does not react with Fehling's reagent. Sucrose is a disaccharide of glucose and
fructose. Most disaccharides are reducing sugars (e.g. lactose and maltose)
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- Sucrose is non-reducing sugar because the anomeric carbon of glucose is involved in the
glucose- fructose bond and hence is not free to form the aldehyde in solution.
Fehling's Reagent: Two solutions are required:
Fehling's "A" uses 7 g CuSO4.5H2O dissolved in distilled water containing 2 drops of dilute
sulfuric acid.
Fehling's "B" uses 35g of potassium tartrate and 12g of NaOH in 100 ml of distilled water.
Method:
1ml test solution + 1ml Fehling's reagent > heat the mixture in BWB (5min)>>Reddish
brown ppt
Experiment-3:
Benedict's Test:
- This test is used also to differentiate between reducing and non reducing sugars.
- It works on the same principle but Benedict is more stable than Fehling's reagent.
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- Benedict's reagent contains blue copper(II) sulfate (CuSO4) · 5H2O which is reduced to red
copper(I) oxide by aldehydes, thus oxidizing the aldehydes to carboxylic acids.
- The copper oxide is insoluble in water and so precipitates. The colour of the final solution
ranges from green to brick red depending on how many of the copper (II) ions are present.
Method:
1ml test solution + 1ml Benedict's reagent > heat the mixture in BWB (5min)>>Reddish
brown ppt
Experiment-4:
Barfoid Test:
- It is a test used to differentiate between monosaccharides and disaccharides. This reaction
will detect reducing monosaccharides in the presence of disaccharides.
This reagent uses
copper ions to detect reducing sugars in an acidic solution. Barfoed's reagent is copper
acetate in dilute acetic acid (pH 4.6)
- Reducing monosaccharides are oxidized by the copper ions in a weak acidic medium to
form a carboxylic acid and a reddish ppt of Cu2O (cuprous oxide).
- Reducing disaccharides (lactose but not sucrose) undergo the same reaction but at slower
rate.
Method:
- 1 ml of the solution to be tested + 2 ml of freshly prepared Barfoed's reagent. Place test
tubes into a boiling water bath and heat for 2 minutes. Remove the tubes from the bath and
allow to cool.
- Formation of a green, red, or yellow precipitate is a positive test for reducing
monosaccharides. Do not heat the tubes longer than 3 minutes, as a positive test can be
obtained with disaccharides if they are heated long enough.
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Experiment-5:
Seliwanoff Test:
- The test reagent dehydrates ketohexoses to form 5-hydroxymethylfurfural. 5hydroxymethylfurfural further reacts with resorcinol present in the test reagent to produce a
red product within two minutes (reaction not shown). Aldohexoses react to form the same
product, but do so more slowly.
Method:
1/2 ml of a sample + 2ml of Seliwanoff's reagent (a solution of resorcinol and HCl) is added.
The solution is then heated in a boiling water bath for two minutes.
- A positive test is indicated by the formation of a red product.
- In case of sucrose, avoid over-boiling because sucrose may be hydrolyzed to its component
(glucose and fructose) and gives false positive result.
-ve
+ve
Experiment-5:
Hydrolysis test:
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- Sucrose is the only non-reducing sugar so it does not reduce the alkaline Cu solutions (Fehling
and Benedict). Sucrose must first be hydrolyzed to its component and then test for.
Method:
6 ml of 1% sucrose in a test tube + 2 drops of concentrated hydrochloric acid (HCl). Heat the tube
in a boiling water bath for 15 minutes.
- Then test for Fehling, Benedict and all the previous tests.
Experiment-6:
Iodine test: Test for Polysaccharides
1- Starch:
- 1/2 mL of the fresh starch solution + 1 drop of the iodine solution.
- A dark blue color indicates a positive test for starch. If the yellow color of the iodine
reagent simply becomes diluted, no starch is present. Record the observation as positive
(blue) or negative (yellow).
2- Dextrin:
- 1/2 mL of the fresh dextrin solution + 1 drop of the iodine solution.
- A violet color indicates a positive test for dextrin. If the yellow color of the iodine reagent
simply becomes diluted, no dextrin is present. Record the observation as positive (violet) or
negative (yellow).
Experiment-6:
The preparation of osazone:
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- Phenyl hydrazine reacts with normal carbonyls to produce phenyl hydrazones.
H
NHNH 2
O
N
N
H+
RCR
RCR
1- Sugars undergo a variation of this reaction in which 3 molecules of phenylhydrazine
react with the sugar to produce a 1,2-diphenylhydrazone.
2- These 1,2-diphenylhydrazones are known as osazones.
H
O
C
CNNH
NHNH 2
H
C
OH
HO
C
H
H
C
H
C
H
3
CNNH
HO
C
H
OH
H
C
OH
OH
H
C
OH
CH 2OH
CH 2OH
osazone
glucose
- Because both carbons 1 and 2 are involved in the reaction C-2 epimers produce the same
osazone.
H
H
O
C
CNNH
NHNH 2
H
HO
C
H
HO
C
H
OH
H
C
OH
OH
H
C
OH
CNNH
H
C
OH
HO
C
H
HO
C
H
HO
C
OH
HO
C
H
C
OH
H
C
CH 2OH
glucose
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3
O
C
NHNH 2
CH 2OH
osazone
3
CH 2OH
mannose
Organized by: Sharifa Al-Ghamdi& Huda Al-Shaibi
Ketoses with configurations identical to aldoses below C-2 give the same osazones e.g.
glucose and fructose.
H
O
C
H
C
CNNH
NHNH 2
OH
3
H
CH 2OH
NHNH 2
C
O
HO
C
H
CNNH
3
HO
C
H
OH
H
C
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH 2OH
CH 2OH
glucose
osazone
CH 2OH
fructose
Explain glucose and fructose form the same osazone?
Characteristics of osazones:
1- Have a characteristic shape.
2- Have characteristic melting points.
3- Specific time and whether the osazone is formed from hot solutions or only on cooling.
- Glucose + Phenyl hydrazine>>>>>>>>>>> Glucosazone (broomed shape)
- Fructose + Phenyl hydrazine>>>>>>>>>>> Fructosazone (broomed shape)
- Maltose + Phenyl hydrazine>>>>>>>>>>> Maltosazone (spherical shape)
- Lactose + Phenyl hydrazine>>>>>>>>>>> Lactosazone
- Sucrose + Phenyl hydrazine>>>>>>>>>>> -ve (WHY?)
Method:
1/2 g of phenyl hydrazine + 1 spoon sodium acetate + 2ml of glucose solution >>>>>BWB (45 min)
until yellow crystals appear >>> cold and examine a sample of crystals under microscope. Compare
the crystal shapes with the supplied photographs.
References:
www.wikipedia.org
www.chemtopics.com
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RESULTS & LAB REPORT
- You are supplied with samples of different sugars carry on all the previous experiments.
- Record your methods, observations and inferences in the three-column format
- Draw the form of crystals.
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SCHEME FOR UNKNOWN SUGAR
Solution of carbohydrate
Benedict’s test
(2ml sugar+2ml Benedicts reagent….boiling 5 min.)
-ve for non reducing sugars
+ve for reducing sugars
Iodine test
Iodine test
(1ml of iodine + 5 drops of sugar)
(1ml of iodine + 5 drops of sugar)
+ve
-ve
Starch
-ve
+ve
Non-reducing
Reducing
Reducing
Disaccharide
monosaccharides
Dextrin
(sucrose)
and disaccharides
*Hydrolysis of sucrose
(5ml of sugar + o.5 ml conc. Hcl
heat in boiling bath for 5 min.
Then perform Fehling, Benedict,
Seliwanoff and Barfoid tests)
Barfoed`s Test
(2ml Barfoed reagent+ 1ml sugar >>boiling 5min)
-ve
+ve
Reducing disaccharides
monosaccharides
(Lactose)
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Seliwanoff`s test
(1ml sugar+1ml Seliwanoff reagent>>>boiling 2 min.)
+ve
Keto-sugar
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-ve
Aldo-sugar
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RESULTS & LAB REPORT
- You are supplied with samples of unknown samples, identify them.
- Record your methods, observations and inferences in the three-column format
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